A fluorescent lamp is one of lamps popularly used in all kinds of lighting systems. In general, a fluorescent lamp includes a light-tranmissive discharge tube, such as glass, quartz, or etc., coated with a fluorescent layer, electrodes for discharging in the discharge tube, and a discharge gas, such as mercury vapor, a buffer gas and etc; sealed in the discharge tube. When electric power is applied to the electrodes, the sealed discharge gas generates discharge and plasma is generated in the discharge tube. Then, electrons in the plasma excite fluorescent materials on the inner surface of the discharge tube to make them emit the visible-light.
As described in the above, because the conventional fluorescent lamp produces plasma in response to discharge produced by the electrodes, the electrodes are deteriorated by the sputtering effect of ions in the plasma. This is known as one of the causes of shortening length of life and limiting input power of a fluorescent lamp.
A conventional electrode-less fluorescent lamp suggested in order to overcome these defects and achieve a fluorescent lamp of long life and high output, is shown in FIG. 1. Generally, an electrode-less fluorescent lamp includes a closed-loop type discharge tube as shown in FIG. 1, cores disposing around on a predetermined region of the closed-loop type discharge tube, and coils wound around the cores.
If predetermined frequency power is supplied to the coil, time-varying magnetic field is induced, therefore plasma inside the discharge tube is produced by induced electromotive force in response to this time-varying magnetic field, and the plasma keeps going unless power is cut off.
Induced electromotive force induced inside the discharge tube is governed by Faraday's law as known well, and the induced electromotive force can be expressed by the following equation.∇×E=−δB/δt  [Math. Equation 1]
This induced electromotive force gives accelerating energy to electrons in the plasma, and the electrons transfer the accelerating energy to particles of other neutral gases by means of collision. According to this, excitation and ionization occur continuously inside the plasma, and it is possible that electric charges are provided continuously by recombination and compensation of surface loss.
In that the visible-light is produced by excitation of a fluorescent layer which is originated from electrons inside plasma produced by inductive coupling, there is no difference from the principle of the conventional fluorescent lamps. Therefore the detailed description is omitted.
As described in the above, in the conventional inductively coupled electrode-less fluorescent lamp, a core disposed around the discharge tube plays not only the same role as a transformer core or a ferrite core, but a role preventing loss of the magnetic field from outside, enhancing power transforming efficiency by keeping the magnetic field inside the core, and enabling to start and maintain the discharge.
Like this, an inductively coupled electrode-less fluorescent lamp does not need electrodes inside the discharge tube. Therefore a fluorescent lamp having a relatively long life can be provided. Because high power can be supplied without worrying about wearing in response to sputtering of electrode materials, a high output fluorescent lamp can be provided. And light-flux decline decreases remarkably because sputtering of a volatile material, such as an electron-radiating material coated on electrode materials or electrodes, does not occur and compound of a volatile material and mercury are not produced.
Until now, various types of electrode-less fluorescent lamps are disclosed. Anderson discloses a closed-loop type tubular electrode-less fluorescent lamp having a discharge current between 0.25 and 1.0 ampere, and a buffer gas pressure between 0.5 and 5 Torr (U.S. Pat. No. 3,500,118). Then, Godyak and etc disclose a closed-loop type tubular electrode-less fluorescent lamp having a mercury vapor and a buffer gas pressure less than about 0.5 Torr unlike Anderson (U.S. Pat. No. 3,500,118). In addition, it is one of the primary features that in operation, the lamp described by Godyak and etc has a discharge current equal to or greater than about 2 amperes in order to achieve higher efficiency than that of Anderson.
As shown in FIG. 2, in order to improve a structure of a simple Anderson-type discharge tube with an excessively heavy core, Godyak and etc fabricate not a discharge tube having a simple closed-loop structure but a discharge tube having a structure necked down on a portion of it, and reduce weight of a lamp assembly in the way that diameter and weight of the core are reduced by disposing it on the portion necked down. In addition, they represent a somewhat complex method for manufacturing the discharge tube.
However, because the electrode-less fluorescent lamp as disclosed in the above has a simple plane structure, light distribution is not uniform. Thus, the lamp is not appropriate to be substituted for an incandescent lamp or a high output lamp used for down lighting used in buildings in an aspect of size and structure. That is, an incandescent lamp or a high output lamp are used for the lighting system, which has a structure put into the ceiling or walls, in most of buildings, and used for local lighting and directional down lighting. The electrode-less fluorescent lamp described in the above has a problem to apply to existing lighting systems because the discharge tube has a plane closed-loop structure.
Further, there are several lamp structures represented to substitute an electrode-less fluorescent lamp for an existing incandescent electric lamp or an existing high output lamp (for example, U.S. Pat. No. 5,767,617, U.S. Pat. No. 5,959,405, and etc), but these approaches are very difficult in manufacturing, thus cause the lamp unit cost to rise.